The present invention relates to the cathode for electron tube which features the electron emission property stabilized for a longer time at a high current density.
Higher definition of the color picture tubes, data display tubes and image pickup tubes has come to require the cathodes of these electron tubes to have electron emission property stabilized for a longer time at high current density.
As a means of meeting these requirements, the following proposals have been made to date.
For example, Japanese Patent Laid-Open 61-271732 and Japanese Patent Laid-Open 62-22347 disclose the dispersing of the powdered scandium oxide in a layer of alkaline earth metal oxide mounted on the base metal. The complex oxide Ba.sub.x Sc.sub.y O.sub.z formed by the reaction between the scandium oxide and the alkali earth metal oxide (e.g. BaO) is dispersed and contained in the electron emissive material, and the complex oxide undergoes gradual thermal decomposition during the operation of the cathode, forming the excess Ba and BaO, which are emitted into the electron emissive material. Therefore, the concentration of the excess Ba and BaO in the layer of the alkaline earth metal oxide is kept high even after the cathode has been operated for a long time, thereby maintaining the excellent electron emission property.
The Japanese Patents Laid-Open 62-90820, 1-311530 and 1-311531 disclose the complex oxide of barium and scandium dispersed in the layer of the alkaline earth metal oxide.
The Japanese Patents Laid-Open 63-310535 and 63-310536 disclose the scandium oxide dispersed in the layer of alkaline earth oxide which is crystallized in prismatic polyhedron or dodecahedron.
The Japanese Patent Laid-Open 62-198029 discloses that two layers of the electron emissive materials are mounted on the base metal, and one layer on the base metal side is provided with dispersed scandium compound.
In the conventional techniques mentioned above, however, consideration has not been given to the stable mass production of the cathodes.
The present inventors have conducted mass production tests to survey the manufacturing of a cathode for an electron tube by the prior technique using the scandium oxide dispersed and impregnated in the layer of the alkaline earth metal oxide. The inventors have found two problems; (1) a problem that a long time is required for aging which is intended to stabilize the electron emission property, and (2) a problem that it is difficult to disperse the scandium oxide uniformly in the layer of the alkaline earth metal oxide at the predetermined ratio, so that the electron emission properties of the respective electron tubes are different from each other.
Problems have also been found out that the powdered complex oxide of the barium and scandium (barium scandate) dispersed in the layer of the alkaline earth metal oxide cannot be made to be dispersed uniformly in an alkaline earth metal carbonate (this is applied on the base metal and is heated in vacuum, to be made into the oxide), so that the content of barium scandate in the layer varies according to each cathode, and the electron emission properties of the respective tubes become different from each other.
In addition, in the cathode the base metal of which is directly coated with barium scandate layer or an alkaline earth metal oxide layer containing dispersed scandium oxide, the bonding strength between the coated layer and the base metal is reduced during the operation of the cathode, and in an extreme case, the coating layer peels off. These phenomena tend to be more conspicuous with the greater amount of the scandium compound contained in said coating layer.